In the field of the semiconductor technology, Insulated-Gate Bipolar Transistor (IGBT) is one of the mainstream large current switch devices. The IGBT has the advantages including high breakdown voltage, and low on-state voltage, etc.
As shown in FIG. 1, the IGBT includes a substrate 1 doped with N-type ions; a gate 2 formed on the front surface S1 of the substrate 1; a P-type well region 3 formed in the substrate 1 and penetrating under the gate 2; and a source 4 doped with N-type ions formed in the P-type well region 3 at one side of the gate 2. The source 4 and the P-type well region 3 are electrically connected by the metal electrode 5; and the gate 2 and the metal electrode 5 are electrically insulated. Further, the IGBT also includes a buffer layer 6 doped with N-type ions formed on the back surface S2 of the substrate 1; and a collector layer 7 doped with P-type ions formed on the buffer layer 6. Along a direction perpendicular to the front surface S1, the P-type well region 3, the substrate 1, and the collector layer 7 form a PNP transistor. The substrate 1 is the wide base of the PNP transistor.
A positive voltage is always applied on the collector layer 7. To turn on the IGBT, a turn-on voltage is applied to the gate 2 to form a channel on the surface of the P well region 3 under the gate 2. Thus, a base current is provided to the base region of the PNP transistor and the transistor is turned on. As shown in FIG. 1, the arrows illustrate the current direction. The doping concentration of the buffer layer 6 is higher than the doping concentration of the substrate 1. Lager number of carriers flow into drift region 1 whose concentration is higher than the intrinsic concentration of the N minus drift region. Thus, the conductance of substrate 1 is greatly reduced, which is called conductance modulation effect; and the current is increased as well. Accordingly, the on-state voltage of the IGBT is reduced. To turn off the IGBT, a turn-off voltage is applied between the gate 2 and the metal electrode 5. The channel disappears, and the IGBT is turned off.
Because the positive voltage is always applied on the collector layer 7, after turning off the IGBT, the PN junction between the P well region 3 and the substrate 1 is reverse biased. When the voltage is higher than the breakdown voltage of the PN junction, the IGBT is damaged.
Reducing the doping concentration of the substrate 1 is able to increase the breakdown voltage (BVDss) of the power device. However, if the doping concentration of the substrate 1 is relatively small, the on-state resistance (Rdson) of the channel region in the substrate 1 is increased; and the on-state voltage of the IGBT is relatively high. Oppositely, increasing the doping concentration of the substrate 1 is able to lower the on-state voltage of the IGBT, but the breakdown voltage of the IGBT will be reduced. Therefore, the on-resistance (Rdson) and the breakdown voltage (BVDss) of the IGBT would have a restriction.
FIG. 2 illustrates an existing approach to break the restriction between the on-state resistance (Rdson) and the breakdown voltage (BVDss) of the IGBT. The approach is referred as super junction technique.
As shown in FIG. 2, a P-type region 8 is formed under the P-type well region 3. The P-type region 8 and the substrate 1 form a PN junction, i.e., super junction. During turning-off, a depletion region is formed by the PN junction between the substrate 1 and the P-type region 8, and fully depleted at last.
FIG. 3 illustrates the electric field distribution in the substrate 1 under the well region 3 of an existing IGBT at off-state without using supper-junction. Ec is the peak value of the electric field. The “y” axis refers to a direction from the first surface S1 to the second surface S2. As shown in FIG. 3, at the off-state of the IGBT, the electric field in the substrate 1 is distributed as a triangle, The peak value of the electric field (Ec) is at P-Well N Drift junction
FIG. 4 illustrates the electric field distribution of an IGBT with a supper junction. The “y” axis refers to a direction from the first surface S1 point to the second surface S2. As shown in FIG. 4, the electric field in the substrate 1 is distributed as a rectangle, the peak value of the electric field (Ec) is away from the interface between the substrate 1 and the P-type well region 3. Comparing with the IGBT without a super junction, during the turning-off process, the depletion region between two adjacent P-type regions 8 is connected when the reverse bias is increased to a certain value. Thus, the electric field in the substrate 1 gradually become a rectangular distribution. Accordingly, the peak value of the electric field is reduced when applying the same collector voltage; and the breakdown voltage of the IGBT with the super junction is increased. At the same time, the on-state voltage of the IGBT with the super junction is not affected. Further, during the turning-off process, the P-type doping regions 8 provide a releasing channel for the carriers near to the channel region of the IGBT. Thus, the switch speed of the IGBT with the super junction is increased.
Because electric field of the IGBT would move to the deeper region of the substrate 1 (toward to the back surface S2), the deeper the super junction is, the higher the breakdown voltage of the IGBT is. However, the conventional ion implantation process is unable to obtain a relative deep P-type doping regions 8 by a single step.
As shown in FIG. 2, an existing method for forming the P-type doping regions 8 includes forming a plurality of stacked epitaxial layers 9. The plurality of the stacked epitaxial layers 9 form the substrate 1. After forming each of the epitaxial layers 9 by a selective epitaxial growth process, a P-type ion implantation is performed on the epitaxial layer 9. After a plurality of selective epitaxial growth processes, the P-type doping regions in the plurality of epitaxial layers 9 stack together; and the P-type doping regions 8 are formed. Thus, the processes for forming the P-type doping regions 8 is relatively complex, and the fabrication time of the P-type doping regions 8 is relatively long. Thus, the production cost of the IGBT is relatively high.
The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems.